Method for nitrogen prefill of high pressure oxygen-containing gas line for gasification

ABSTRACT

The present invention provides a much safer solution to pressurizing oxygen-containing gas feed lines at the start up of an oxygen-containing gas using process unit. All piping downstream of the main oxygen-containing gas block valve is pressurized with inert nitrogen prior to introducing the oxygen-containing gas into the piping. By equalizing with nitrogen it removes the hazard of slowly pressurizing the piping with the oxygen-containing gas and exposing the oxygen-containing gas to the substantial pressure drop associated with pressurizing piping to a high pressure.

BACKGROUND OF THE INVENTION

[0001] The process and advantages of gasifying hydrocarbonaceous material into synthesis gas, or syngas, are generally known in the industry. In high temperature gasification processes, synthesis gas is commonly produced from gaseous combustible fuels, such as natural gas, and solid combustible organic fuels, such as coal, residual petroleum, wood, tar sand, shale oil, and municipal, agriculture or industrial waste. The gaseous or solid combustible organic fuels are reacted with a reactive oxygen-containing gas, such as air, enriched air, or pure oxygen, and a temperature modifier, such as steam, in a gasification reactor to obtain the synthesis gas.

[0002] The generation of enriched air or pure oxygen may be carried out by an air separation unit (ASU). The transfer of the oxygen from the air separation unit to the gasifier must be done under high pressure because of the generally high temperature and pressure conditions of the gasifier.

[0003] All organic or inorganic material may react with gaseous or liquid oxygen at certain pressure and temperature conditions. The reaction that occurs can cause a fire or explosion due to the uncontrolled oxidation of the material in the presence of gaseous or liquid oxygen. Because of these inherent dangers, the design of the piping and the process of start-up and shut down must be carefully considered.

[0004] Many materials that are used in pipes and control valves have ignition temperatures that are above the normal flowing temperature of gaseous oxygen. Thus the ignition of these materials in the presence of an oxygen-containing gas is not the danger. The danger is in the ignition of these materials by localized high temperatures. Several factors may cause localized high temperatures including, foreign particle impingement, vibration and flow velocity. Ignition by foreign particle impingement is caused by particles, such as weld slag, being carried along by the flow of the oxygen-containing gas. When these particles encounter a valve or other obstruction, such as a bend in the pipe, the particle may collide with the valve or the pipe. This collision could transform the particle's kinetic energy into sufficient heat to cause the particle or the surface that it strikes to ignite. Ignition caused by vibration may occur in a part that is vibrating sufficiently to generate internal friction that raises the temperature above its ignition point. For this reason clearances and guiding are important in oxygen-containing gas service lines. Flow velocity becomes an important matter when high-pressure oxygen-containing gas is used or large volumes of oxygen-containing gas must be transferred. In general, if the velocity through a pipe or valve exceeds 200 ft/s, the friction caused by the walls of the pipe or the valve may result in auto-ignition of the material. Velocity criteria for oxygen service are outlined in the Compressed Gas Association Pamphlet G-4.4 the contents of which are incorporated by reference.

[0005] In prior art processes of pressurizing oxygen-containing gas feed lines at the startup of a gasifier, it was required to equalize the pressure across the oxygen-containing gas feed block valve. Previously bleed valves have been used to slowly bleed the high pressure oxygen-containing gas into the oxygen-containing gas feed piping by throttling the block valve. Because of the large pressure drop associated with pressurizing the feed piping, this practice requires many safety upgrades, such as expensive metallurgy for use in the valves and piping to prevent auto-ignition of the oxygen, fire shields, or remote valve actuators. In the case of a valve failure, a large rush of oxygen-containing gas at a large pressure could easily cause a catastrophic incident. It would thus be desirable to develop a safer process for pressurizing oxygen-containing gas feed piping in anticipation of starting up a gasifier.

SUMMARY OF THE INVENTION

[0006] The present invention provides a much safer solution to pressurizing oxygen-containing gas feed lines at the start up of an oxygen-containing gas using process unit. All piping downstream of the main oxygen-containing gas block valve is pressurized with inert gas, preferably nitrogen prior to introducing oxygen-containing gas into the piping. By equalizing the pressure with an inert gas it removes the hazard of slowly pressurizing the piping with oxygen-containing gas and exposing the oxygen-containing gas to the substantial pressure drop associated with pressurizing piping to a high pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 shows a common prior art process for pressurizing oxygen-containing gas feed lines.

[0008]FIG. 2 illustrates a preferred embodiment of the present invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0009] Referring now to FIG. 1, a prior art process of pressurizing an oxygen-containing gas feed line to an oxygen-containing gas using process unit is shown. A common oxygen-containing gas using process unit is a gasification reactor, but can be any oxygen-containing gas using process unit. First, oxygen-containing gas from a main oxygen-containing gas header (not shown) is supplied in line 1. Block valve 2 blocks the oxygen-containing gas from flowing into the oxygen-containing gas feed piping and eventually into the gasifier while the gasifier is shut down.

[0010] When the gasifier is started up and the oxygen-containing gas line has to be placed it service, equalization valve 5 is slowly opened so that oxygen-containing gas can flow into line 3. When this is done, the oxygen-containing gas flowing through equalization valve 5 sees a large pressure drop. Thus, the oxygen-containing gas is traveling at a speed in excess of 200 ft/s, increasing the risk of auto-ignition as the gas passes through equalization 2 and is abruptly stopped by control valve 7. Because Line 3 is at low pressure, it is desirable to place block valve 2 and equalization valve 5 close to control valve 7 to minimize the amount of piping the high pressure oxygen-containing gas is released into. After line 3 is pressurized with oxygen, control valve 7 is then opened up so as to feed oxygen-containing gas to the gasifier 8. Some prior art processes do not use an equalization valve, opting instead to use the much larger block valve to pressurize the oxygen-containing gas feed piping.

[0011] A preferred embodiment of the present invention is illustrated in FIG. 2. For simplicity sake, common elements from FIG. 1 have been given the same number. Oxygen-containing gas from a main header (not shown) is supplied via line 1 and is prevented from entering the gasifier feed piping system by block valve 2. When the gasifier is ready for startup, the gasifier and the oxygen-containing gas feed piping system are first purged with an inert gas. As used herein, an inert gas is any gas that will not react with any materials in the downstream process units. A preferred inert gas is nitrogen.

[0012] When it becomes desirable to pressurize the oxygen-containing gas feed piping, inert gas through line 9 is used to pressurize the gasifier feed piping system. This is done by sending pressurized inert gas through line 12 and block valve 13 into oxygen-containing gas feed pipe 6. Once the oxygen-containing gas feed piping system is sufficiently pressurized, control valve 7 is opened to send pressurized inert gas into piping section 3 upstream of control valve 7. Once the inert gas pressure is substantially equivalent to that of the oxygen-containing gas in line 1, control valve 7 is closed to isolate the inert gas. Equalization valve 5 and block valve 2 can be opened, and oxygen-containing gas will flow into the oxygen-containing gas feed piping system.

[0013] When describing the inert gas pressure as being substantially equivalent to that of the oxygen containing gas, what is meant is that the pressure of the inert gas is such that when the equalization valve 5 and the block valve 2 is opened, the oxygen will flow through the valves at a velocity less than 200 ft/s. This has the effect of reducing the friction caused by the walls of the pipe or the valve that would otherwise increase the possibility of auto-ignition of the material.

[0014] Sophisticated interlocks can also be used to control the method of pressurizing the oxygen-containing gas feed piping system. These interlocks will also ensure that the oxygen-containing gas feed piping system is pressurized safely upon gasifier startup. These interlocks will perform a variety of functions. The interlocks also verify that the gasifier is safely shutdown and that all purges have been completed, and then verify that the gasifier pressure and oxygen-containing gas feed piping pressure is low, preferably below 20 psi. The interlocks will then verify that the oxygen-containing gas feed piping is isolated and that the nitrogen pressure is adequate to pressurize the oxygen-containing gas feed piping and to protect the nitrogen system. In addition, the interlocks will also verify that upstream oxygen-containing gas pressure is available when it comes time to open the nitrogen block valve so as to protect the oxygen system. If all conditions are present the interlock system will allow the oxygen-containing gas feed piping to be pressurized. Once pressurized, the upstream oxygen-containing gas equalization valve and block valve can be opened.

[0015] The above illustrative embodiments are intended to serve as simplified schematic diagrams of potential embodiments of the present invention. One of ordinary skill in the art of chemical engineering should understand and appreciate that specific details of any particular embodiment may be different and will depend upon the location and needs of the system under consideration. All such layouts, schematic alternatives, and embodiments capable of achieving the present invention are considered to be within the capabilities of a person having skill in the art and thus within the scope of the present invention.

[0016] In view of the above, one of ordinary skill in the art should appreciate that one illustrative embodiment of the present invention includes a method for reducing frictional combustion in an oxygen-containing gas line. This method comprises purging the oxygen-containing gas line with an inert gas, pressurizing the oxygen-containing gas line with the inert gas to a pressure equivalent to the pressure of the oxygen-containing gas being introduced, and introducing oxygen-containing gas into said oxygen-containing gas line. Preferably, the inert gas is nitrogen, and the oxygen-containing gas is air, enriched air, or pure oxygen.

[0017] The pressurized oxygen-containing gas line preferably contains a oxygen-containing gas block valve upstream of an oxygen-containing gas control valve. The oxygen-containing gas control valve in turn is upstream of an oxygen-containing gas using process unit. When pressurizing such a line, the inert gas is introduced between the oxygen-containing gas control valve and the oxygen-containing gas using process unit. The oxygen-containing gas control valve is then opened so as to pressurize the piping between the control valve and the oxygen-containing gas block valve. After pressurizing that piping, the oxygen-containing gas control valve is closed, isolating the gas in the piping upstream of that control valve. The introduction of the oxygen-containing gas into the oxygen-containing gas line is the accomplished by opening the oxygen-containing gas equalization valve, and later opening the oxygen-containing gas block valve.

[0018] A plurality of interlocks can also be used to help control this process. For example, one of said interlocks verifies can be used to verify that the purging the oxygen-containing gas line with an inert gas is complete. Another of the interlocks can be used to verify that the inert gas pressure is adequate to gas to the pressure the oxygen-containing gas line to a pressure equivalent to that of the oxygen-containing gas being introduced. An interlock can be used to verify that the oxygen-containing gas line is isolated. Furthermore, one of said interlocks may be used to verify that the pressure of the source of the oxygen-containing gas is at a sufficient pressure so that after the oxygen-containing gas line is pressurized with the inert gas and the oxygen-containing gas is introduced into the oxygen-containing gas line, the inert gas will not flow back into the source of the oxygen-containing gas.

[0019] This method is useful when used in conjunction with an oxygen-containing gas feed line to a gasification reactor. A method for use with a gasification reactor would comprise the steps of purging the oxygen-containing gas feed line with an inert gas, pressurizing the oxygen-containing gas feed line with the inert gas to a pressure equivalent to the pressure of oxygen-containing gas being introduced, and introducing the oxygen-containing gas into the oxygen-containing gas line. The preferred inert gas is nitrogen, and the preferred oxygen containing gasses are air, enriched air, or pure oxygen.

[0020] A plurality of interlocks can also be used to help control the feed of an oxygen containing gas to a gasification reactor. For example, one of said interlocks verifies can be used to verify that the purging the oxygen-containing gas line with an inert gas is complete. Another of the interlocks can be used to verify that the inert gas pressure is adequate to gas to the pressure the oxygen-containing gas line to a pressure equivalent to that of the oxygen-containing gas being introduced. An interlock can be used to verify that the oxygen-containing gas line is isolated. Furthermore, one of said interlocks may be used to verify that the pressure of the source of the oxygen-containing gas is at a sufficient pressure so that after the oxygen-containing gas line is pressurized with the inert gas and the oxygen-containing gas is introduced into the oxygen-containing gas line, the inert gas will not flow back into the source of the oxygen-containing gas.

[0021] While the apparatus and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the process described herein without departing from the concept and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the scope and concept of the invention. In particular, it should be noted that although the preferred embodiments were described using a gasification reactor as the oxygen-containing gas using process unit, the apparatus and methods of this invention can be used for any oxygen-containing gas using process unit. 

What is claimed is:
 1. A method for reducing frictional combustion in an oxygen-containing gas line, the method comprising: purging the oxygen-containing gas line with an inert gas; pressurizing the oxygen-containing gas line with the inert gas to a pressure equivalent to the pressure of the oxygen-containing gas being introduced; and introducing oxygen-containing gas into said oxygen-containing gas line.
 2. The method of claim 1, wherein the inert gas is nitrogen.
 3. The method of claim 1, wherein the pressurized oxygen-containing gas line is an oxygen-containing gas feed line to a gasification reactor.
 4. The method of claim 1, wherein the oxygen-containing gas is selected from the group consisting of air, enriched air, and pure oxygen.
 5. The method of claim 1, wherein the pressurized oxygen-containing gas line comprises a oxygen-containing gas block valve upstream of an oxygen-containing gas control valve, the oxygen-containing gas control valve being upstream of an oxygen-containing gas using process unit.
 6. The method of claim 5, wherein the oxygen-containing gas using process unit is a gasification reactor.
 7. The method of claim 5, wherein the oxygen-containing gas line is pressurized with the inert gas between the oxygen-containing gas control valve and the oxygen-containing gas using process unit.
 8. The method of claim 6, wherein the oxygen-containing gas control valve is opened so as to pressurize a section of the oxygen-containing gas line between the oxygen-containing gas block valve and the oxygen containing gas control valve.
 9. The method of claim 8, wherein the introduction of the oxygen-containing gas into said oxygen-containing gas line is accomplished by opening an oxygen-containing gas equalization valve that bypasses the oxygen-containing gas block valve.
 10. The process of claim 1, further comprising using a plurality of interlocks to control the pressurizing of the oxygen-containing gas line with the inert gas.
 11. The process of claim 10, wherein one of said interlocks verifies that the purging the oxygen-containing gas line with an inert gas is complete.
 12. The process of claim 10, wherein one of said interlocks verifies that the inert gas pressure is adequate to gas to the pressure the oxygen-containing gas line to a pressure equivalent to that of the oxygen-containing gas being introduced.
 13. The process of claim 10, wherein one of said interlocks verifies that the oxygen-containing gas line is isolated.
 14. The process of claim 10, wherein one of said interlocks verifies that the pressure of the source of the oxygen-containing gas is at a sufficient pressure so that after the oxygen-containing gas line is pressurized with the inert gas and the oxygen-containing gas is introduced into the oxygen-containing gas line, the inert gas will not flow back into the source of the oxygen-containing gas.
 15. A method for placing an oxygen-containing gas feed line to a gasification reactor in service, the method comprising: purging the oxygen-containing gas feed line with an inert gas; pressurizing the oxygen-containing gas feed line with the inert gas to a pressure equivalent to the pressure of oxygen-containing gas being introduced; and introducing the oxygen-containing gas into the oxygen-containing gas line.
 16. The method of claim 15, wherein the inert gas is nitrogen.
 17. The method of claim 15, wherein the oxygen-containing gas is selected from the group consisting of air, enriched air, and pure oxygen.
 18. The process of claim 15, further comprising using a plurality of interlocks to control the pressurizing of the oxygen-containing gas feed line with the inert gas.
 19. The process of claim 18, wherein one of said interlocks verifies that the purging the oxygen-containing gas feed line with an inert gas is complete.
 20. The process of claim 18, wherein one of said interlocks verifies that the inert gas pressure is adequate to gas to the pressure the oxygen-containing gas feed line to a pressure equivalent to that of the oxygen-containing gas being introduced.
 21. The process of claim 18, wherein one of said interlocks verifies that the oxygen-containing gas feed line is isolated.
 22. The process of claim 18, wherein one of said interlocks verifies that the pressure of the source of the oxygen-containing gas is at a sufficient pressure so that after the oxygen-containing gas feed line is pressurized with the inert gas and the oxygen-containing gas is introduced into the oxygen-containing gas feed line, the inert gas will not flow back into the source of the oxygen-containing gas. 